The present invention relates to wind-powered generators, and more specifically to wind-powered alternators to be operated at a fixed frequency.
Large-diameter wind-powered alternators, which typically utilize a 4-pole alternator and operate at slow wind-rotor speeds, utilize speed increasers, with a gear ratio such as 1:45, to drive the alternator at snychronous speed, producing alternating current in synchronization with an electric utility grid. A small change in rotor speed caused by wind gusts and lulls, mulitiplied by the speed increaser, may force the alternator out of synchronization, causing it to experience torque reversals which are unsuitable for reliable power generation and which might destroy the alternator. Even more minute changes in rotor speed, as caused by gravitational forces on the somewhat flexible rotor blades, and the wind wake as the wind rotor rotates near the windmill support tower, cause unsuitable current spikes.
Attempted solutions to these problems have included use of slip clutches, torsionally flexible drive shafts and oversized alternators capable of absorbing and resisting the sudden increased torque and power delivered by the wind rotor. Wind-powered alternator systems with adjustable rotor blade pitch may be used to achieve alternator synchronization, but cannot accommodate sudden changes in rotor shaft speed due to the substantial interval required to make pitch changes. Control systems with wind-speed sensors upwind of the rotor which anticipate wind-speed changes have not been found to be adequate to correct for sudden gusts and lulls, and cannot compensate for cyclic perturbations in rotor shaft speed due to gravitational forces on the rotor or the tower wake.